In humans, errors in meiosis I are the leading cause of birth defects, mental retardation, and a significant contributor to infertility. In meiosis I, chromosomes exhibit a unique segregation pattern; sister chromatids remain together, while homologous chromosomes segregate from each other. Human aneuploidy sometimes occurs because sister chromatids separate in meiosis I, as they do in mitosis. Other times aneuploidy occurs because homologous partners segregate to the same side of the spindle at meiosis I. The correct meiotic segregation pattern is achieved by meiosis-specific mechanisms that alter chromosome structures and the cell cycle control of segregation. First, meiosis I segregation is accomplished by tethering the homologous chromosomes through recombination so they will act as segregation partners. Second, kinetochores are altered, and centromeric cohesins are protected from removal, so sister chromatids remain joined. Third, a previously unrecognized process, homologous centromere pairing (CEN-pairing), holds the centomeres together in ways that help them orient on the spindle. Fourth, the assembly of the spindle is delayed so the formation of meiotic chromosome pairs can be completed before the chromosomes begin segregating. Failures in any of these meiotic modifications to chromosome behavior could be culprits in human meiotic segregation errors. The experiments proposed here focus on these meiosis-specific processes and adress: How are meiosis I kinetochores assembled so that sister chromatids will segregate as a unit? What is the basis for CEN-pairing? How is spindle assembly coordinated with chromosome behavior to allow proper completion of the re-structuring of meiotic centromeres? The proposal is divided into three sets of experiments. The experiments use budding yeast as a model organism and a combination of molecular genetic, cell imaging and biochemical approaches. The first set of experiments will explore how the cell disassembles the kineotchores that are on chromosomes when meiosis begins, and replaces them with kinetochores that will dictate meiosis I segregation behaviors. These experiments will employ mass spectrometry methods that will allow assessment of overall kinetochore composition as cells progress through meiosis I, and complementary imaging approaches that will examine behaviors of individual kinetochore components. The second set of experiments will examine how CEN-pairing is accomplished. CEN-pairing requires the synaptonemal complex (SC) protein, Zip1, which persists at the paired centromeres after SC disassembly. The final set of experiments employs genetic methods to determine how chromosome and spindle dynamics are coordinated in meiosis so that spindle formation is blocked until chromosomes have properly identified and paired with their segregation partners. Completion of the proposed experiments will elucidate how meiotic remodeling of kinetochore structures, and centromere and spindle behaviors, allows homologous chromosomes to be segregated with high fidelity in meiosis I.